This application is not related to any pending applications.
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1. Technical Field of the Invention
This invention relates to a lens attachment system for an aircraft
2. Prior Art
Navigational lights on aircraft are typically located along the leading edge of the wings. These lights are covered by a lens which is shaped to conform to the surface of the aircraft to which it is attached. Aircraft owners have historically had problems maintaining the lenses.
While in service, the lenses are exposed to extreme forces such as erosion from hail, sleet, sand and the like. The erosion deteriorates the translucent nature of the lens, thus reducing the effectiveness of the underlying lights.
Thermal expansion also contributes greatly to the failure of the lenses. The temperature range for the exterior of an aircraft can range from 110xc2x0 F. at ground level on an airport tarmac to xe2x88x9260xc2x0 F. at a cruising altitude of 30,000 feet. The problem of the wide temperature ranges is compounded by the fact that the coefficient of thermal expansion for a typical lens is approximately three times the coefficient of thermal expansion of aluminum, the primary material use for aircraft airframes. The repeated cycling of this thermal expansion causes the lenses to fail. The typical mode of failure is that the area between the bushing and the edge of the lens develops fatigue cracks.
Other factors contribute to this failure. Lenses are manufactured to the exact dimensions of the airplane as it was originally manufactured. However, once the airplane is flown, the exact dimensions are changed slightly by the forces exerted on the aircraft during flight. As such, a replacement lens will not precisely fit an aircraft. In order to replace that lens, the mechanic must exert additional forces on the lens in order to align the fastener holes and attach it to the airframe. This introduces residual stresses on the lens which further contribute to the fatigue of the lens.
The lenses must be constructed out of material which is capable of withstanding these extreme service conditions and residual stresses. The lenses have been known to be made out of polycarbonate, as well as acrylic. Other aircraft lens manufacturers have attempted to solve this fatigue cracking problem by reinforcing the edge of the lens with a fiberglass composite. However, this solution may introduce erosion problems, as well as adding unnecessary cost to the manufacturer of the lens.
U.S. Pat. No. 5,366,577 issued to Robert N. Hart and David M. Trebes on Nov. 22, 1994 discloses a method of manufacturing a lens for use as a part of an aircraft including the steps of fusion bonding a sheet of acrylic to a sheet of polycarbonate to form a sheet of laminate, cutting the laminate into a blank having a configuration required by the ultimate lens configuration, heating the blank and forming it around a mold to provide a three dimensional lens body of desired size and configuration having an uninterrupted circumferential edge and having an inner polycarbonate surface and an outer acrylic surface, applying a narrow strip seal to the lens body inner surface in a narrow strip portion adjacent substantially the full circumferential edge thereof in order to bond the strip seal to the lens body, shaping the circumferential edge of the lens body around substantially the full circumferential edge thereof, and providing bushings in the lens body and strip seal so the bushings are spaced around the circumferential edge as a means of securing the lens to an aircraft
Even with these advances in materials and manufacturing methods failure of aircraft lenses remains a problem.
Another drawback to the lenses and lens attachment systems known in the field is the excessive amount of time it can take to install a lens cover. In order to attach the lens to the aircraft the fastener holes in the lens must be aligned with the fastener holes in the aircraft. As previously mentioned, this is complicated by the fact that the fastener holes in the lens are located according to the dimensions of the original design. The actual dimensions of the fastener holes of the aircraft are altered from the dimensions of the original design due to the forces exerted on the aircraft while in flight. The end result is that these two sets of fastener holes do not always align properly. The problem is further complicated by the gasket. The holes in the gasket must also be aligned with the corresponding fastener holes in the lens in aircraft. As can be seen, this can become a very labor intensive repair.
The present invention solves the problem of fatigue cracking in aircraft lenses by using a unique and novel lens attachment system. It is an objective of the present invention to provide a lens attachment system capable of absorbing some of the thermal and residual stresses inherent with replacement aircraft lens.
It is also an objective of the present invention to provide a lens attachment system which will reduce the amount of time necessary to replace an aircraft lens.
The lens attachment system of the present invention is comprised of a plurality of keyways intersecting the edge of the lens which eliminate the area susceptible to cracking. A gasket comprised of a resilient material is then used to secure the lens to the airframe. The gasket has a plurality of keys which correspond to the keyways of the lens. The gasket and lens are then attached to the airframe by a plurality of fasteners which extend through the keys.
The lens is attached to the aircraft through the interference of the keys with the keyways. The gasket and keys are made of a resilient material. The resiliency of the gasket and keys allows more flexibility when attaching the lens to the aircraft. This flexibility cases the alignment process of the fastener holes.
A better understanding of the invention will be obtained from the attached specification, drawings and claims.